One conventional example method for performing color separation of an image signal to obtain a signal for a colorant to be used by a printing apparatus, such as a printer, is shown in FIG. 49.
FIG. 49 is a diagram showing the configuration of an image system that performs color separation. R, G and B represent red, green and blue image signals, respectively. Further, C, M, Y, K, Lc and Lm represent cyan, magenta, yellow, black, light cyan and light magenta signals, respectively. A control unit 4901 performs a predetermined color separation process, and converts image signals R0, G0 and B0 and image signals C0, M0, Y0 and K0, which are received from a personal computer, into data for ink C1, M1, Y1 and K1, which are colorants used, for example, by a printer.
In a color separation process, when R0, G0 and B0 are signals indicating monitor colors, such as sRGB, the signals are processed, so that colors reproduced on the monitor are the same as colors reproduced by an image forming apparatus.
Furthermore, when C0, M0, Y0 and K0 are signals indicating colors reproduced through image printing, the signals are processed, so that colors reproduced through image printing are the same as colors reproduced by the image forming apparatus.
Sequentially, a cyan colorant dark and light separation unit 4902 performs color separation and separates a signal value C1 for a cyan colorant into a colorant value C3, for dark cyan, and colorant value Lc3, for light cyan. Similarly, a magenta colorant dark and light separation unit 4903 performs color separation, and separates a signal value M1, for a magenta colorant, into a colorant value M3, for dark magenta, and a colorant value Lm3, for light magenta.
Through this processing, a color separation table for three inputs of R, G and B and six outputs of C, M, Y, K, Lc and Lm can be prepared (see, for example, patent documents 1, 2 and 3).
In addition, patent document 4 discloses a method whereby a LUT is employed to convert data for four colors, of cyan, magenta, yellow and black, into data for a total of seven colors, dark and light cyan, dark and light magenta, dark and light yellow and black.
However, in the above described process, one-dimensional dark and light color separation is independently performed for cyan and magenta colorants. Therefore, depending on the colorant and the characteristic of a printing medium, the total colorant amount, an total amount for all the colorants, would exceed an amount limit, and image quality would be deteriorated.
That is, in a case wherein the total colorant amount for C, M, Y, K, Lc and Lm exceeds a colorant amount total limit for an electrophotographic printer, for example, an image may not be fixed to a printing medium and may peel off. Further, in a case for an inkjet printer, ink may not be absorbed by a printing medium and may run.
To cope with this, a process for correcting colorant amount is performed.
FIG. 50 is a diagram for explaining the correction of colorant amount, and showing an example relationship between a cyan colorant input amount and cyan dark and light colorant output amount. The horizontal axis represents the cyan colorant input amount, and the vertical axis represents the cyan dark and light colorant output amount.
For a case wherein the total colorant amount (the total for the cyan dark and light colorant output amount) exceeds a limitation value (e.g., A %), the total colorant amount can be decreased simply by reducing the cyan colorant input amount.
However, when a cyan colorant input amount reduction percentage ranges from 80% to 100%, the cyan dark and light colorant amount are, to the contrary, increased. Therefore, when processing is performed to reduce the cyan dark and light colorant amount output, a case may be encountered wherein the cyan colorant input amount must not only be reduced to a value equal to, or smaller, than a limitation value, but to a percentage lower than 80%, at which the total cyan dark and light colorant amount is the largest. At this time, in the above described case wherein one-dimensional color dark and light separation is independently performed, the amount of light ink would be increased, and the density would be reduced.
Further, since the process for correcting the colorant amount is performed separately for cyan and magenta, optimal color separation can not be performed while taking into account the amount of four colorants, cyan, magenta, yellow and black.
Thus, as one problem, granularity and the consumed colorant amount can not be controlled by adjusting the amount of a light colorant across an entire color area.
Furthermore, patent document 4 discloses a method whereby a LUT is employed to convert data for four colors, cyan, magenta, yellow and black, into data for a total of seven colors, dark and light cyan, dark and light magenta, dark and light yellow and black. However, a specific preparation method for the LUT is not described.
Patent Document 1: Japanese Patent Application Laid-Open No. 6-226998
Patent Document 2: Japanese Patent Application Laid-Open No. 9-163161
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-230020
Patent Document 4: Japanese Patent Application Laid-Open No. 10-098625
Patent Document 5: Japanese Patent Application Laid-Open No. 2003-116016